The present invention relates to a picture image photographing apparatus using a solid-state imaging device, and more particularly, to an improved apparatus for preventing a picture image from deteriorating due to optical shot noise occurring in a solid-state imaging device.
In recent years, there has been extreme improvements in solid-state imaging devices and picture image photographing apparatus which uses solid-state imaging devices as an input portion which can perform high-speed scanning of an image having a high resolution. Solid-state imaging devices applied to picture image photographing apparatus, are classified into the projection type using a reduction optical system, and the contact type using a rod-lens system. The most realistic apparatus to realize high resolution scanning, such as over 600 DPI (dots per inch) scanning, is those that adopt projection type sensors, and this is, at present, due to the limitation of the resolution of optical systems. There is shown in FIG. 1 a conventional picture image photographing apparatus using such a reduction optical system. In FIG. 1, the conventional apparatus comprises a solid-state imaging device 2 having a line sensor 3, and an imaging lens 4 for projecting on the line sensor 3 a picture image of a line 5 on a paper 1 to be photographed. Then, as shown in FIG. 2, every imaging element 11 of the solid-state imaging element changes the picture image into signal charges corresponding to the quantity of light, and a CCD (charge coupled device) register 12 reads out the signal charges. The read-out signal charges are then converted into a voltage by an output buffer 13 to output outside the device 2 through an output terminal 14. It is possible to read information on the paper 1 by scanning in the direction of an arrow 6 to the paper 1.
The conventional image photographing apparatus, as described above, has the problems of the deterioration of signal-noise ratio caused by optical shot noise, especially when the apparatus is improved to be actuated for high speed and high resolution. It is well, known that a number of electrons which are read from a imaging element may be represented by a normal distribution of a standard deviation (N.sub.av).sup.1/2 when the average number of electrons contained in the charge pocket is N.sub.av. Furthermore, N.sub.av may be described as follows: EQU N.sub.av =A.times.S.times.T.times.I,
where A denotes a constant, S denotes an area of the imaging element, T denotes a time for reading informations of one line (integration time), and I denotes a quantity of the light incident to the imaging element. An integral time for converting an image into the signal charges, is set to be equal to a read-out time T for one line.
As described above, even though the same light irradiates and the same integral time is set, numbers of the read-out electrons may be different from each measuring time. This phenomenon is named as as an optical shot noise and causes the deterioration of a signal to noise ratio.
The signal to noise ratio restricted by this phenomenon may be represented as follows: EQU S/N=(N.sub.av).sup.1/2 / N.sub.av =1/ (A.times.S.times.T.times.I).sup.1/2
Furthermore, the realization of high-speed and high resolution requires a decrease of both the imaging element area S and the integration time T, thereby reducing the number of electrons occurring under the same quantity of the light. If a light quantity I increases with a reduced amount of the imaging element area S and integration time T, it is possible to keep the same S/N in principle. However, because the light quantity I has a limit to increase in actuality, there is an essential limit of the deterioration of S/N caused by the optical shot noise when the apparatus is modified to be high speed and high resolution.
If it is only an improvement of the S/N caused by the optical shot noise, the same line sensor 3 may scan the same line 5 on the paper 1 at a plurality of times and the read-out data may be averaged for each same imaging element. Namely, the same line is scanned by n times to generate an average signal having an average value N.sub.av and the standard deviation (N.sub.av).sup.1/2, thereby improving an n.sup.1/2 time of an S/N in comparison with a case scanning only one time. However, because the same line should be scanned n times, it takes n times the read-out time for the paper, thereby extremely reducing the high-speed efficiency.